In communication systems based on e.g. Code Divisional Multiple Access (CDMA), power control is used to meet the desired quality of service targets. The power control may be implemented both in the user equipment to meet the downlink quality target and also in the base station to meet the uplink quality target. In wireless communication networks, the downlink is the transmission path from the base station to the user equipment, and the uplink is the transmission path from the user equipment to the base station. It is important that the power control is able to maintain the desired quality of service target despite varying radio conditions, which is often the case in wireless communication systems.
The following describes various technical aspects related to inner loop power control, outer loop power control and its convergence in CDMA systems. In particular the methods and devices described herein relates to Wideband Code Division Multiple Access (WCDMA) but may be equally applicable to other CDMA based technologies such as e.g. cdma2000 because power control, both inner and outer loop, is the hallmark of CDMA access technology. The methods may also be implemented in HSPA, and in particular the uplink variant of HSPA, also called Enhanced Up-Link (EUL) or HSUPA.
In CDMA systems the inner loop power control, also called fast power control, runs every time slot, which is typically less than 1 ms (e.g. 0.67 ms in WCDMA). In WCDMA the inner loop power control runs in both uplink and downlink. The fast inner-loop power control adjusts the transmit power of the sender towards a specific Signal to Interference and noise Ratio (SIR) target at the receiver. The aim of the uplink and downlink inner loop power controls is to counter the effect of fast fading, while maintaining the desired SIR target. In the uplink the power control also compensates for the near-far problem, so that the signal received from the users far out in the cell are not swamped out by the users' close to the base stations stronger signals. During every slot the user equipment estimates the SIR on some known reference or pilot symbols and compares it with some SIR target corresponding to the given service (e.g. Block Error Rate (BLER), certain Bit Error Rate (BER) requirements and spreading factor used etc.). In WCDMA, Downlink SIR is measured on Dedicated Physical Control Channel (DPCCH), which comprises pilot bits and Transmitter Power Control (TPC) bits. The TPC commands are also used for uplink power control and the pilot bits are primarily used for channel estimations. If the estimated SIR is less than the SIR target then the user equipment generates UP command, otherwise it generates DOWN command; in response the base station will increase (in case of UP) or decrease (in case of DOWN) its downlink transmit power.
The aim of the outer loop power control is to adjust the SIR target value used by the inner loop power control as previously explained, while maintaining a certain link quality. The quality target (e.g. BLER of the data) is set by the network and is expected from the user equipment to consistently maintain this target to ensure the desired quality of service is met throughout the session. Due to the varying radio link conditions e.g. user mobility, fast fading etc, the mapping between the SIR target and BLER changes over time. This is a key point as it requires frequent adjustment of the SIR target to maintain the desired value of BLER. This mechanism of adjusting the SIR target is also referred to as outer loop power control, quality control or outer loop scheme.
In systems such as enhanced uplink (EUL) version of WCDMA, the outer loop power control is configured to fulfill a quality target based on number of transmission attempts i.e.: “after x targeted transmissions, the residual block error rate should be y %”.
The uplink outer loop power control for enhanced uplink channels adjusts the uplink DPCCH SIR target so the residual error rate after the stipulated maximum number of transmissions is fulfilled.
If the transmission is not successfully decoded after the stipulated maximum number of transmissions, the SIR target is increased by e.g. 0.5 dB. For every successfully decoded transmission, the corresponding SIR target is decreased by a factor inversely proportional to the error probability, e.g. about 0.01 dB if the error rate is 2%.
The transmission of data over the air in a wireless communication system is performed by using a plurality of different physical channels, for example Dedicated Physical Control CHannel (DPCCH), Dedicated Physical Data CHannel (DPDCH), Enhanced Dedicated Physical Control CHannel (E-DPCCH) and Enhanced Dedicated Physical Data CHannel (E-DPDCH). The power consumptions of these are related to each other by power offsets, i.e. β-values or gain factor relative the power level of the DPCCH.
However, the gain factor used in actual data transmission may be inaccurate, which in turn will affect the overall system performance. Either the reference gain factors obtained through simulations or the method to calculate other gain factors may result in inaccurate gain factors. E.g. when the gain factor is lower than required, more transmission attempts are required to guarantee the successful transmission. Since current EUL outer loop power control is based on transmission attempts, this actually means the SIR target is increased and more power is allocated to DPCCH. However, this is undesired.
A state-of-art implementation of the outer loop power control for EUL WCDMA increases the DPCCH SIR target when the number of transmission attempts is larger than TA target. This means that all other channels with a power offset to the DPCCH, such as the E-DPDCH, will also increase their transmit power.
However, in many cases, the reason for not fulfilling the TA target is due to too low power on the data channel, E-DPDCH, not due to too low power on the control channel DPCCH.
Since the DPCCH is continuously transmitted while E-DPDCH is transmitted more intermittently, increasing the SIR target and power on the DPCCH causes unnecessarily high interference.
Moreover, in a situation with bad coverage and high power usage on the user equipment, DPCCH will “steal” power from the data channel E-DPDCH.
Further on, there is also a problem with the constant power offsets for different transport block size sizes. It is difficult for the network to set the power offset and the enhanced data channel transport format combination (E-TFC) to match the TA target exactly. For example if the power offset is too low to fulfill the TA target, the DPCCH SIR will be increased until the TA target is fulfilled. Since it may be difficult to know beforehand what power offset can match the wanted TA target, this will most likely lead to an unwanted adjustment of the DPCCH SIR target, and thereby lead to a possibly unwanted interference increase or a too low DPCCH SIR target.